Polymers formed from polymerized units of thiophene and substituted thiophene monomers which possess relatively low band gaps (Eg) demonstrate measurable electrical conductivity. Such polymers are often referred to as intrinsically conducting polymers. The term, band gap (Eg), refers to the energy difference between electronic energy levels called the conduction band and the valence band. The band gap exhibited by a given polymer depends upon a variety of factors including the structure of the monomer making up the polymer. For example, polythiophene demonstrates a band gap of 2.1 eV, poly (2-decylthieno[3,4-b]thiophene) demonstrates a band gap of 0.92 eV and poly(2-phenylthieno[3,4-b]thiophene) demonstrates a band gap of 0.85 eV.
Intrinsically conducting polymers consisting solely of aromatic repeat units in the polymer backbone are typically not soluble in water. Consequently, such polymers are typically processed using organic solvents. Several methods have been employed to increase the solubility of intrinsically conducting polymers in various organic solvents. Such methods include (1) forming a derivative of the monomer to increase the solubility of the side chains of the monomer in a given organic solvent; (2) modifying the polymer backbone by employing oligomeric conjugated systems and flexible spacers; and (3) using charge compensating dopants.
U.S. Pat. No. 5,300,575 (the '575 patent) discloses dispersions of polythiophenes which are suitable for use as antistatic coatings for plastic moldings. These polythiophenes are prepared by polymerizing the corresponding monomer in the presence of oxidizing agents typically used for the oxidative polymerization of pyrrole and/or with oxygen or air in the presence of a polyanion. The polythiophenes of the '575 patent have a relatively low Eg of 1.7 eV compared to poly(thiophene) which has an Eg of 2.1 eV.
The polythiophenes of the '575 patent are typically prepared by polymerizing 3,4-ethylenedioxythiophene in the presence of poly(styrene sulfonic acid). The resulting linear polymer is purified using both anion and cation exchange resins wherein poly(styrene sulfonate) serves as a charge compensating dopant. The resulting polymer forms a colloidal dispersion in water because poly(styrene sulfonate) is soluble in water and demonstrates a strong ionic interaction with the cationic polymeric backbone.
Those skilled in the art to which this invention pertains continue to search for intrinsically conducting polymers which exhibit useful bandgaps for industrial applications, which can be readily dispersed in water and which are stable in solution to afford a useful shelf life.